IPC: Message Passing

Notes

• A far more flexible method for communications is message passing
• In general
  • send(message), receive(message)
  • Works if there is a single sender/receiver
  • Unix pipes work like this.
• Pipes
  • This is a two way data communications mechanism.
  • But generally one process reads and another writes.
  • Pipes are created with the int pipe(int pipefd[2]); command.
  • fd[0] is the output end (STDIN_FILENO)
  • fd[1] is the intput end (STDOUT_FILENO)
  • See pipePi.cpp.
• Pipes used this way are tied to a comomon ancestor
• But named pipes exist
  • mknod filename p
  • And process can open/close this as well.
• Pipes are
  • limited to a single machine.
  • Have no distinct messages
  • Have no priorities, are essentially bit streams.
• Message queues
  • Allow processes to send messages between each other.
  • But not directed at a process, just a queue
  • Messages have boundaries, and priorities
  • And, as the name implies are queued
• There are both posix and system V message queues
• Again, we will look at posix message queues
  • Four maintenance functions

    • mqd_t mq_open(const char *name, int oflag, mode_t mode,
                           struct mq_attr *attr);

      • name must start with /
      • interface is created in /dev/mqueue
      • oflags like files
        • O_RDONLY, OWRONLY, ORDWR
        • plus O_CREAt, O_EXCL, O_NONBLOCK, O_CLOEXEC
      • mode only if creating
      • attr only if creating.

        • struct mq_attr {
                         long mq_flags;       /* Flags (ignored for mq_open()) */
                         long mq_maxmsg;      /* Max. # of messages on queue */
                         long mq_msgsize;     /* Max. message size (bytes) */
                         long mq_curmsgs;     /* # of messages currently in queue
                                                 (ignored for mq_open()) */
                     };

        • can be nullptr
        • seems only needed when trying to make the values greater than the default.
  • int mq_getattr(mqd_t mqdes, struct mq_attr *attr);
    • and int mq_setattr(mqd_t mqdes, const struct mq_attr *restrict newattr, struct mq_attr *restrict oldattr);
    • Gets and sets the attributes for the queue.
  • int mq_close(mqd_t mqdes);
  • int mq_unlink(const char *name);
• Sending and receiving messages
  • int mq_send( mqd_t mqdes, const char msg_ptr[msg_len], size_t msg_len, unsigned int msg_prio);
  • int mq_receive(mqd_t mqdes, const char msg_ptr[msg_len], size_t msg_len, unsigned int msg_prio);
  • There are also thread safe versions, but that is for later.
• You can also
  • Set an interrupt like condition for when a new message arrives with mq_notify
  • Send a message with a timeout with mq_timedsend
  • And a timed receive mq_timedrecive
• Look at writer.cpp and reader.cpp
  • I am only sending ascii messages, but I could also send raw data.
• There is more to message queues, but this will do for this class.